sharepoint on prem profile incremental crawl not working

Hi,

In my sharepoint on prem installation I use windows form authentication and the accounts are synced with AD. 

Everything works fine except foe the incremental crawl. 

Whe I add new users to AD the incremental crawl doesn't sync the new users, I have to run a full crawl in order to get the new users synced med sharepoint. 

What I am missing? 

UPDATE I have deleted the entire User Profile service and recreated it but the issue still. I have checked that the account has replicating direcotry changes in AD.

Best regards

Americo